MESSAI.iO

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72% confidence

This paper reports isolation and electrochemical characterization of two subsurface Betaproteobacteria (Delftia and Azonexus) enriched from a deep Death Valley aquifer using electrodes poised at 272–572 mV vs. SHE as electron acceptors. Anodic currents developed within 20 days coupled to acetate oxidation; the enriched community was dominated by Comamonadaceae and Rhodocyclaceae families known as denitrifiers. Pure cultures of Delftia (WE1-13) and Azonexus (WE2-4) confirmed extracellular electron transfer (EET) ability at 522 mV vs. SHE, with modest protein accumulation. The work demonstrates that electrode-based enrichment can reveal electrochemical activity in organisms previously known only for alternate metabolisms like nitrate reduction.

Key insights

  • Threshold Anodic currents developed within 20 days across all four electrode potentials (272–572 mV vs. SHE), with acetate concentration declining from 10 mM to 7.5 mM over one month, indicating broad potential tolerance for acetate-oxidizing EET.

    So what. Operators should expect rapid biofilm establishment (3 weeks) across a wide potential window when enriching acetate-oxidizing anode communities from subsurface inocula; potential selectivity may be weak for these organisms.

  • Mechanism Current declined abruptly with every medium change, suggesting a role for soluble redox shuttles (indirect EET) rather than direct cell-surface electron transfer.

    So what. Maintain continuous medium supply without batch replacements if direct EET is desired; batch operation may inadvertently select for shuttle-dependent organisms.

  • Mechanism SEM imaging revealed extracellular filaments morphologically similar to bacterial nanowires, despite the organisms being Betaproteobacteria (not the canonical Geobacter/Shewanella model systems).

    So what. Do not assume nanowire-like structures are absent in non-model organisms; subsurface Betaproteobacteria may employ diverse EET mechanisms beyond those in laboratory strains.

  • Method Delftia and Azonexus were previously known only as denitrifiers and chlorate reducers, but electrochemical testing at 522 mV vs. SHE confirmed they can use electrodes as electron acceptors—a 'hidden' metabolism not revealed by traditional cultivation.

    So what. Screen subsurface isolates electrochemically even if their known metabolism (e.g., denitrification) is already characterized; electrode-based assays reveal metabolic versatility missed by conventional media.

  • Performance Protein content increased only modestly despite rising anodic current, suggesting low biomass yield or slow growth kinetics on electrodes.

    So what. Do not rely on protein or cell count as a proxy for electrode activity in these organisms; measure current directly and independently validate biomass.

Opportunity areas6

Specific gaps in this paper, with an experiment that would close each.

  • High No measurement of nitrogen species (NO3−, NO2−, N2O, NH4+) in the enrichment medium or effluent despite enriching denitrifiers (Comamonadaceae, Rhodocyclaceae) known to reduce nitrate.

    The paper states these families are 'short-chain fatty acid-utilising denitrifiers' and 'known to reduce chlorate and iron', yet no nitrogen cycling data are reported. The enrichment medium contained NH4Cl and (NH4)2SO4 as nitrogen sources, but no analysis of whether nitrate was present, consumed, or produced.

    Try. Measure NO3−, NO2−, N2O, and NH4+ in influent and effluent during electrochemical enrichment at each potential (272–572 mV vs. SHE); determine if denitrification occurs in parallel with anode respiration or is suppressed by electrode availability.

  • High No systematic variation of electrode potential during pure culture testing to map the electrochemical activity window for Delftia and Azonexus.

    Enrichment used four potentials (272–572 mV vs. SHE), but pure culture testing fixed the electrode at 522 mV vs. SHE only. No polarization curves or potential-dependent current measurements are provided.

    Try. Perform chronoamperometry or linear sweep voltammetry on Delftia WE1-13 and Azonexus WE2-4 across 200–700 mV vs. SHE to identify optimal potential, onset potential, and maximum current density for each strain.

  • High No measurement of current density, power density, or Coulombic efficiency; only absolute current reported.

    Figure 2 shows current (µA) over time but electrode area is 3 × 2 cm = 6 cm². No normalization to area or calculation of current density (µA/cm²) or power density (mW/m²) is provided, limiting comparison to other MES studies.

    Try. Normalize all current measurements to electrode area; calculate current density (µA/cm²) and Coulombic efficiency (moles electrons transferred / moles acetate oxidized) for both enrichment and pure culture experiments.

  • Medium No direct measurement of electron transfer mechanism (direct vs. indirect EET) despite speculation about redox shuttles and nanowires.

    Authors observe current decline with medium change and SEM filaments, but do not test for soluble redox mediators (e.g., flavins, quinones) or use electrochemical impedance spectroscopy (EIS) to distinguish direct and indirect pathways.

    Try. Quantify extracellular flavins or quinones in culture supernatant; perform EIS on biofilm-coated electrodes to estimate charge transfer resistance; test if current recovers after medium change if mediators are re-added.

  • Medium No testing of alternative electron acceptors (nitrate, chlorate, Fe(III)) in parallel with electrode respiration to determine metabolic hierarchy or competition.

    The enriched organisms are known denitrifiers and chlorate reducers, yet the paper does not test whether they preferentially use electrodes over soluble acceptors, or whether both pathways operate simultaneously.

    Try. Perform batch experiments with Delftia and Azonexus in NDW2 medium containing acetate + electrode (at 522 mV vs. SHE) + NO3− (or Cl−) at varying concentrations; measure current and nitrogen species to determine acceptor preference.

  • Medium No characterization of biofilm structure, thickness, or cell density on electrodes; only SEM images and protein content reported.

    Protein content increased 'modestly' but absolute values and cell counts per electrode are not provided. Biofilm architecture and its role in current generation are not quantified.

    Try. Measure biofilm thickness by confocal laser scanning microscopy (CLSM); quantify viable cell density (CFU/cm² or DAPI counts) on electrodes at different time points; correlate with current density.

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What they did

System
MFC

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Abstract

Abstract Continental subsurface environments can present significant energetic challenges to the resident microorganisms. While these environments are geologically diverse, potentially allowing energy harvesting by microorganisms that catalyze redox reactions, many of the abundant electron donors and acceptors are insoluble and therefore not directly bioavailable. Extracellular electron transfer (EET) is a metabolic strategy that microorganisms can deploy to meet the challenges of interacting with redox-active surfaces. Though mechanistically characterized in a few metal-reducing bacteria, the role, extent, and diversity of EET in subsurface ecosystems remains unclear. Since this process can be mimicked on electrode surfaces, it opens the door to electrochemical techniques to enrich for and quantify the activities of environmental microorganisms in situ . Here, we report the electrochemical enrichment of microorganisms from a deep fractured-rock aquifer in Death Valley, California, USA. In experiments performed in mesocosms containing a synthetic medium based on aquifer chemistry, four working electrodes were poised at different redox potentials (272, 373, 472, 572 mV vs. SHE) to serve as electron acceptors, resulting in anodic currents coupled to the oxidation of acetate during enrichment. The anodes were dominated by Betaproteobacteria from the families Comamonadaceae and Rhodocyclaceae. A representative of each dominant family was subsequently isolated from electrode-associated biomass. The EET abilities of the isolated Delftia strain (designated WE1–13) and Azonexus strain (designated WE2–4) were confirmed in electrochemical reactors using working electrodes poised at 522 mV vs. SHE. The rise in anodic current upon inoculation was correlated with a modest increase in total protein content. Both genera have been previously observed in mixed communities of microbial fuel cell enrichments, but this is the first direct measurement of their electrochemical activity. While alternate metabolisms (e.g. nitrate reduction) by these organisms were previously known, our observations suggest that additional ‘hidden’ interactions with external electron acceptors are also possible. Electrochemical approaches are well positioned to dissect such extracellular interactions that may be prevalent in the subsurface.

Keywords

MicroorganismRedoxMicrobial fuel cellEnvironmental chemistryBetaproteobacteriaAnode

Identifiers

DOI
10.1101/046979
Journal
bioRxiv (Cold Spring Harbor Laboratory)
Year
2016
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